Case series / case reportindependent cross-checkfull text checked★ key study I-FAST-112 in-depth review
Side effects of supervised water-only fasting
Is fasting safe? A chart review of adverse events during medically supervised, water-only fasting.
Overview
Finnell JS et al.. BMC Complement Altern Med 2018 · PMID 29458369 · DOI 10.1186/s12906-018-2136-6
Among 768 supervised water-only fasts, about one in four had a severe (grade 3) side effect; two people needed hospital care.
Does the study support the specific claim? No · Methodological quality: low
Plain-language summary
Study explained simply
- What did the researchers want to find out?
Describe adverse events of water-only fasting.
- How was the study done?
Chart review of 768 clinic visits.
- How many people, animals or samples were studied?
Adults at a US fasting centre.
- What was actually found?
72.3% mild/moderate highest AE; 27.6% grade 3; 2 SAEs.
- How large was the observed effect?
Cumulative incidence grade>=3 AE: 0.28 at day 10.
- What do the results mean in everyday life?
Even with medical supervision and water, multi-day fasting often causes significant side effects.
- What are the study's limitations?
Retrospective; clinic-run; conflicts of interest.
- What does it say about Elawady's specific claim?
Contradicts claims that multi-day fasting strengthens the body.
- Do the results apply to Germany and Europe?
Applies; same physiology.
- What can explicitly not be concluded from this study?
Outcomes of unsupervised fasting at home.
Scientific in-depth review
1. Full reference
- Authors
- Finnell JS et al.
- Journal
- BMC Complement Altern Med
- Year
- 2018
- Identifier
- PMID 29458369 DOI 10.1186/s12906-018-2136-6
- Verification
- verified – PubMed efetch 2026-10-26; Crossref update-to/updated-by check
- Retracted
- no
- Corrections
- none
- Extent checked
- full text
- Conflicts of interest and funding
Senior author directs the fasting centre; two authors paid consultants to the centre's foundation; funded by the National Health Association (stated in full text).
2. Research question
What adverse events occur during medically supervised water-only fasting?
3. Study design and population
- Study design
- Case series / case report – Retrospective chart review
- Population
Adults >=21 years water-only fasting >=2 days at a residential facility
- Country
USA (California)
- Period
2006-2011
- Sample size
768 visits (of 2539 charts)
- Inclusion and exclusion criteria
Water-only fast >=2 consecutive days with refeeding of half the fast length
4. Methods and endpoints
- Exposure / intervention
Medically supervised water-only fasting
- Comparator
none
- Endpoints
Adverse events (CTCAE grading)
- Statistical methods
Descriptive; Kaplan-Meier cumulative incidence
5. Actual results
- Results
Highest AE grade <=2 in 555 visits (72.3%), grade 3 in 212 visits (stated 26.6%; recalculated 27.6%), grade 4 in 1, no deaths. 5961 AEs, 75.3% grade 1. Estimated cumulative incidence of grade >=3 AE: 0.20 at day 5, 0.28 at day 10, 0.32 at day 15. Two SAEs requiring hospitalisation: grade-3 dehydration on day 3 (73-year-old man) and grade-4 hyponatraemia on day 9 (70-year-old man). Common AEs: fatigue, insomnia, headache, hypertension, presyncope (28.3% of visits).
- Absolute risks
not reported
- Authors' conclusion
Most AEs were mild to moderate and known reactions to fasting; the protocol can be safely implemented in a medical setting with minimal risk of SAE.
6. Strengths
- Large number of supervised fasts
- Standardised AE grading
- Full text read
7. Weaknesses
- Retrospective, single centre, single abstractor
- Conflicts of interest
- SAE percentage misreported (0.002% instead of about 0.26%)
8. Risk of bias
- Orienting tool
- none applicable
- Type of assessment
- simplified assessment (abstract only)
- Full formal assessment
- no
- Overall risk of bias
- high
- Rationale
Retrospective case series with conflicts of interest.
9. Assessment of the statistical interpretation
About 1 in 4 to 1 in 3 fasts had a grade-3 adverse event by day 10-15 even under supervision; SAE proportion 2/768 = 0.26% (paper's 0.002% is an error). See stats/I-FAST-112_finnell_ae.py.
10. Relevance to the specific claim
Shows that even supervised water-only fasts (with water) produce frequent adverse events, including dehydration and severe hyponatraemia - contradicting 'not eating for days makes the body strong'.
11. Independent contrary findings
12. Transferability to Germany and Europe
Universal biology; applies in Europe and Egypt.
13. Overall methodological judgement
Contradicts the claim; the authors' reassuring conclusion is stronger than their own grade-3 rates justify.
| Multidimensional assessment | |
|---|---|
| Authenticity of the source | verified |
| Methodological quality | low |
| Risk of bias | high |
| Precision of results | moderate |
| Directness | direct |
| Reproducibility | not replicated |
| Consistency with the overall evidence | consistent |
| Regional transferability | high |
| Adequacy of the interpretation (by the archive) | partly correct but over-interpreted |
14. Open questions
15. Linked fact checks
- Health equals the ability to avoid toxic inputs: zero inputs means infinite health; complete abstinence from all inputs (fasting) keeps the body in a state that does not become ill, and not eating for three or four days makes the body strong. – Studies contradicting or limiting the claim
- Do not drink much water because it distends the abdomen and raises abdominal pressure; even when thirsty during long fasting hours just wet the lips; fasting three days in a row each week without food and drink is not harsh. – Studies contradicting or limiting the claim